Proportioning temperature control apparatus

ABSTRACT

A proportioning temperature control apparatus provides for a variable width temperature band over which proportional control is effected, and wherein variation of the bandwidth automatically varies the rate at which proportional control is effected at various points within the band, the rate decreasing with decreasing bandwidth. The variable bandwidth is effected most advantageously by means of a unique control circuit which utilizes a capacitor whose voltage is added or otherwise mixed with the output of a means for generating a progressively varying voltage as the temperature of the environment being controlled varies within the control band. A capacitor charge circuit selectively controls the application of a pair of variable DC voltages to the capacitor depending upon the presence or absence of heating or cooling signals in the output of the control apparatus. These DC voltages are selected to be near the voltage values to which the capacitor must be charged to effect the turnon and turnoff of the heating or cooling producing signals involved. Variation of one or the other of the DC voltages referred to automatically varies the control bandwidth and the rate of control.

United States Patent I i 13,s91,077

[72] Inventors Ahdor ll. Alton Primary Examiner-William E. Wayner LakeZurich; Anomeywallenstein, Spangenberg, l-lattis and Strampel Michell l.Kohn, Wheeling, both of, ill. [21] Appl. No. 827,714

23 rj z ABSTRACT: A proportioning temperature control apparatus A i neea lndusmes Inc provides for a variable width temperature band over which55 g Memchen NJ proportional control is effected, and wherein variationof the bandwidth automatically varies the rate at which proportionalcontrol is effected at various points within the band, the rate [54]PROPORTIONING TEMPERATURE CONTROL decreasing With decreasing bandwidth.The variable band- APPARATUS width is effected most advantageously bymeans of a unique 19 claims, Drawing m control circuit which utilizes acapacitor whose voltage is added or otherwise mixed with the output of ameans for [52] [1.5.01 236/69,

generating a progressively varying voltage as the temperature 165/26219/502 236/78 307/265 317/1335 of the environment being controlledvaries within the control {5 l] Int. Cl 605d 23/22, band A capacitorcharge circuit selectively controls the appli 605d 1 1/28 cation of apair of variable DC voltages to the capacitor de- [50] Field of Search236/78, 69,

pending upon the presence or absence of heating or cooling 1 C; 165/26;219/499' 494307510165; signals in the output of the control apparatus.These DC volt- 317/13351 124; 73/355 ages are selected to be near thevoltage values to which the capacitor must be charged to effect theturn-on and tumofi' of [56] References cued the heating or coolingproducing signals involved. Variation of UNITED STATES PATENTS one orthe other of the DC voltages referred to automatically 3,240,428 3/1966Umrath 236/78 varies the control bandwidth and the rate of control.

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I 21 250 my Max Max. I I I 40 my Hm Bio/v0 wwm 40 l Zz"=4mv Max MM L=40m1/ MIN BAND wmm M645. mv/r 38-3 yra lOFF 25-3 l gei 104D 11 23 I I JI 30 JE BRIDGE VOLT/96E M c/RculT 500Ac5 FORM/N6 v Mew/v.5 l I 219 i J li PATENTEU JUL 5 1921 SHEET 2 OF 8 hem 4%?" 1 /442 -MLZWZZ 11/60 NN NIEtu N I l9" M ywm, ilezflz ij f PATENTED JUL 6197! SHEET 3 BF 8 E $8 QEEImm? mml WHEEL. jkq 5s QQQQQQQU D PROPORTIONING TEMPERATURE CONTROAPPARATUS more particularly to proportioning temperature controlapparatus. it has application to systems which control temperaturethrough heating or cooling the environment involved,

although the particular exemplary application of the invention to bedescribed will be a heating system.

Proportioning temperature control systems are desirable because, unlikeon-off temperature control systems where the temperature of theenvironment being controlled oscillates between limits, the temperatureof the controlled environment can be held perfectly constant after ashort delay period when the system becomes stabilized. in the case wherethe ambient temperature of the environment is lower than the controltemperature and the control temperature is reached by the energizationof a heater coil or the like, as the ambient temperature remains belowthe lowest temperature of the band of temperatures over whichproportional control is to be effected,

the heating coil is generally energized continuously. When the lowerlimit of this temperature band is reached, proportional control isinitiated whereupon the heating coil is cyclically energized anddeenergized with the ratio of the power on" (i.e., heat on) to the power"off" intervals each cycle being very large, the ratio progressivelydiminishing to approximately one as the temperature increases to themidpoint of the band and further diminishing to where the ratio of thepower on" to the power off intervals is very small when the temperaturereaches the upper limit of the control band. For temperatures above thecontrol band, the heater coil is substantially continuously deenergized.(Such a proportioning control system anticipates and corrects forthermal inertia by preventing control point overshoot on mostinstallations, with the resultant quick stabilization of thetemperature.)

The band of temperatures over which proportional control is obtained issometimes made variable because the bandwidth for best temperaturecontrol varies with the particular environmental conditions involved.The user of the proportioning temperature control apparatus involvedadjusts a setpoint control to select a given temperature, usually thetemperature at the midpoint of the temperature control band involved, at

or near the desired environmental temperature. However, the

actual stabilized temperature which is obtained, although falling withinthe control band involved, generally differs somewhat from the setpointtemperature in the absence of an automatic offset eliminating means, thedifference in these two temperatures being commonly referred to astemperature offset" or droop." A proportioning control system having arelatively wide control band naturally produces a much greater offsetthan a system adjusted for a relatively narrow control band. A narrowcontrol band however, is disadvantageous where the environmentalconditions vary rapidly or where it is desired to obtain temperaturestabilization as quickly as possible.

The frequency at which the temperature control power is cyclicallyturned on and off has been found to be an important factor for effectiveproportional control. This has not been generally appreciated, with theresult that it is generally the case that a given proportioning controlsystem has only a .,.limited number of applications where it can be mosteffectively applied. It has been discovered that, when proportionalcontrol its effected over a wide bandwidth, the frequency ofinterruption of the power on" and power off" signals should generally berelatively high and, when proportional control apparatus is effectedover a relatively narrow bandwidth, the frequency of interruption of thepower on" and power off" signals should be relatively low. Also, thefrequency of the interruption rate of the power on and power off"signals of a system adjusted to a given bandwidth should progressivelyvary from a minimum value where the temperature is at the extremes ofthe band to a maximum at the center portion of the band where rapidclose control is needed. The operation of a wide bandwidth proportioningcontrol system with a relatively low interruption rate at a given pointwithin the band produces a control system where it is difficultfrequently to stabilize the temperature. The operation of a narrowbandwidth proportioning control system with a relatively highinterruption rate at a given point within the band frequently producestemperature drift.

Most, if not all prior proportioning temperature control systems do notprovide for the ready variation of both bandwidth and rate and they arenot designed with the appreciation of the advantageous relationshipbetween bandwidth and frequency described above. Moreover, because ofthe nature of the proportioning control circuitry used, the temperatureof the controlled environment indicated by the apparatus often variedwith the ambient temperature (i.e., the cold junction of thethermocouple commonly used to sense the environmental temperature).

One of the aspects of the present invention is the provision ofproportioning control apparatus with the provision of means forselectively varying the control bandwidth while automaticallysimultaneously varying the frequency of the power on" and power off"interruption rate so that the rate decreases with the control bandwidth.

Another aspect of the invention is the provision of proportioningtemperature control apparatus with a simple, reliable circuit forcontrolling the interruption of the power on" and power ofi" signals andwhich is readily adjustable to vary both the control bandwidth and theinterruption rate, most advantageously together, so that an increase inthe bandwidth automatically increases the interruption rate. Also,unlike the prior proportioning control systems where the circuit usedfor proportional control made it difficult to provide compensation forcold junction or ambient temperature variation, the preferred forms ofthe invention separate electrically the temperature measuring circuitfrom the proportional control circuit so that cold junction compensationcan be easily provided to provide a direct reading of temperature whichis independent of ambient temperature conditions.

In the most advantageous form of the invention, the proportioningcontrol circuit uses a simple capacitor charge circuit which develops acyclic voltage across a capacitor which is added or mixed with theoutput of a means which generates a voltage which varies progressivelywith the environmental temperature within the control band. Thecapacitor charge circuit controls the application of a pair of variableDC voltages to the capacitor depending upon the presence or absence ofheating or cooling signals in the output of the control apparatus. TheseDC voltages when respectively added to the output of said voltagegenerating means when the temperature of the environment beingcontrolled is at the respective limits of the desired control bandprovide resultant voltages which are at or near voltage values whichrespectively effect the turn on and turnoff of heating or coolingproducing signals. Variation of one or the other of the DC voltagesreferred to automatically varies the control bandwidth and the rate ofcontrol.

The above and other advantages and features of the invention will becomeapparent upon making reference to the specification to follow, theclaims and the drawings wherein:

FIG. 1 is a simplified, schematic diagram of the basic components usedin a heating application of the present invention;

FIG. 2 illustrates a temperature versus time curve provided by anyproportioning control apparatus including that of the present invention;I

FIG. 3 is an enlarged view of the measuring unit forming part of theapparatus shown in FIG. 1;

FIG. 4 illustrates the heating signals generated by the control unitforming part of the apparatus of FIG. 1 when the temperature of thecontrol environment is at the extremes and midpoint of a relatively widecontrol band;

FIG. 5 is a view corresponding to FIG. 4 with the proportional controlapparatus of FIG. I adjusted to operate over a relatively narrow controlband;

FIG. 6 is a schematic diagram, partly in block form, illustrating thecircuits of the measuring and control units shown in FIG. 1;

FIGS. 7A, 7B and 7C illustrate the voltage waveforms developed acrossthe capacitor forming part of the circuitry shown in FIG. 6 when theenvironmental temperature is respectively at the low end, the midpointand the upper end of the control band when the circuitry is adjusted toprovide the widest control band;

FIG. 8 illustrates capacitor charge curves where a capacitor charges todifferent voltages;

FIGS. 9A, 9B and 9C shows the corresponding voltage waveforms developedacross the capacitor in FIG. 6 when the circuitry is adjusted to providethe narrowest control band;

FIG. 10 illustrates an exemplary circuitry of the proportional controlsystem illustrated in FIG. 6;

FIG. 11 is a circuit diagram of an alternative form of the inventionwhich has a number of advantages over the circuit of FIG. 10-,

FIG. HA is a circuit diagram of the capacitor charge circuit formingpart of the circuit of FIG. II when the heating or cooling signalcontrol relay is energized;

FIG. 12B is a simplified circuit of FIG. 12A;

FIG. 13A is a circuit diagram of the capacitor charge circuit formingpart of the circuit of FIG. ll when the heating or cooling signalcontrol relay is deenergized;

FIG. 13B is a simplified circuit of FIG. 13A;

FIG. 14 is a modified capacitor charge circuit which provides for theautomatic elimination of any offset between the set point temperatureand the actual temperature of the environment; and

FIGS. 15 a -e is a diagrammatic view illustrating the manner in whichthe circuit of FIG. 14 operates.

Referring now to FIG. 1, the proportioning control system shown thereinand generally indicated by reference numeral 1 includes a measuring unit2 which has, among other things; a temperature indicating scale 3 shownin enlarged form in FIG. 3 across which moves a temperature indicatingpointer 6 which assumes a position on the scale 4 which indicates thetemperature of the environment whose temperature is to be controlled.Also movable across the scale 4 by operation of a manually operablecontrol knob 8 is a setpoint indicating member 10 whose tip is setopposite the number on the scale 4 indicating approximately the desiredtemperature for the environment being controlled. The temperature of theenvironment to be controlled may be detected by a suitable thermocouplell or other input means which is connected to a measuring circuitforming part of the measuring unit 2 which operates a galvanometermovement to which the index pointer 6 is attached.

The temperature of the environment in which the thermocouple I1 islocated is shown controlled by a heater coil 12 which, within theproportioning control range of the system, cyclically receives currentpulsations cyclically applied thereto whose average value produces thedesired tempera.- ture of the environment involved under control of acontrol unit 14. The control unit 14 receives a DC voltagefrom themeasuring unit 2 which indicates whether or not the index pointer 6 isbelow or above the limits of the temperature band over whichproportional control is effected or is within this control band. In thelatter case, the amplitude of the voltage progressively varies as theindex pointer 6 moves between the limits of the widest such control bandover which the apparatus is adjustable. For temperature below the lowesttemperature of this control band, the voltage fed to the control unitfrom the measuring unit may be a constant relatively high value and fortemperatures above the upper limit of the control band involved thisvoltage may be a fixed relatively low value. Such a range of voltagevariations may be readily provided, for example, by a light responsiveelement like a photocell or solar cell arrangement such as illustratedin FIG. 6. When the temperature of the environment to be controlled isbelow the lower limit of the control band involved, the control unit 14feeds a continuous current to the heater coil 12. When the temperatureof the environment reaches the low limit of the control band involved,the current fed to the heater coil will be cyclically interrupted forvery short intervals to produce current pulses as indicated by currentpulse waveform P1 in FIG. 4 illustrating the operation of the system forthe widest control band. As indicated, the period of each cycle for thepulses P1 is relatively long (eg 40 seconds) 'with the interruption ofthe current pulses occurring for only a very small fraction of thisperiod. As the temperature of the environment progressively increasesfrom the lower limit of the control band involved, the ratio of thepower on" to the power off" interval of each cycle progressivelydecreases as the interruption rate progressively increases to a pointwhere this ratio is 1 at approximately the midpoint of the control bandinvolved as indicated by the current pulses P2 in FIG. 4. As an example,the period between successive current interruptions may be approximately12 seconds with the power on" and power off intervals approximatelyequal when the period of each cycle is 40 seconds under the aforesaidconditions illustrated in FIG. 4. As the temperature of the environmentfurther progressively increases toward the end of the control bandinvolved, the ratio of the power on" to the power off" intervals of eachcycle progressively decreases and the current interruption rateprogressively decreases. At the upper end of the control bandillustrated in FIG. 4, very short current pulses P3 are fed to theheater coil with the period between current pulsations being relativelylong (e.g. 40 seconds). 1

As shown in FIG. 6, the setpoint indicator member 10 may be part of amoving frame which carries a light responsive element 18 on one portionof the frame and a light source- 20 on another portion of the frame sothat light is directed to the light responsive element 18. Thetemperature indicating pointer 6 attached to a galvanometer movement 22which carries a masking element 24 which, at the low temperature end ofthe widest control band referred to, begins to move into the path of thelight beam passing from the light source 20 to the light responsiveelement 18. The masking element 24 progressively blocks off this lightbeam in progressively increasing degrees as the temperature indicatingpointer 6 moves across the widest control band. At the upper temperaturelimit of the control band, the masking element 24 substantiallycompletely blocks the light beam. The solar cell 18 may act as avariable impedance element as in the case of a photocell or voltagegenerating element to provide a variable DC output in accordance withthe degree to which the light beam is inter cepted by the maskingelement 24.

In accordance with one of the aspects of the present invention, thewidth of the temperature control band is electrically varied by means ofa single control member 13 (FIG. I.) in the control unit 14. Thus, inone extreme position of the bandwidth control member 13, the band oftemperature over which proportional control is obtained may be, forexample, 4 Fahrenheit, and, as this control member is progressivelymoved to its other extreme position, the control band varies, forexample, to a maximum of 40 Fahrenheit. Also, as the bandwidth controlmember 13 is adjusted, in a manner soon to be explained, theinterruption rate of the heating signal for temperatures within thecontrol band involved is most advantageously varied so that theinterruption rate at a particular location of the control bandprogressively decreases with decreased bandwidth.

FIG. 5 illustrates an example of the effect of the adjustment of thebandwidth control member 13 on the current pulsations fed by the controlunit 14 to the heater coil 12 when the bandwidth control member 13 isadjusted to provide an exemplary narrow control band. As illustrated,the narrow width control band is only 4 Fahrenheit. When the temperatureof the environment reaches the lower end of this control band, currentpulses P1 are fed to the heater coil at a lower frequency than thecurrent pulses Pl for the widest bandwidth operation, with the intervalof each current pulsation occupying substantially the entire periodexcept for a small interruption interval as indicated. The periodbetween successive current interruptions then may, for example, be 45seconds. As the temperature of the environment progressively increasesto the midpoint of the band, the decrease in the period and increase inthe interruption rate of current pulsations P2 relative to that duringthe widest band operation becomes less pronounced. For exam ple, theperiod between current pulsations P2 may be 20 seconds rather than 12seconds for the widest band operation. The power on" and power offintervals at the midpoint of the narrowest control band areapproximately equal as in the case of the wider band operation. When thetemperature of the environment reaches the upper limit of the narrowestcontrol band, as illustrated, the current pulses P3 are very short so asto occupy only a small portion of each cycle. The exemplary period maythen be approximately 45 seconds.

In the proportioning control system as illustrated, the variation of thetemperature of the environment with time is illustrated by the curve C1in FIG. 2 where, after a few cycles of temperature oscillation, thesystem will end up with a ratio of power on to power off each cyclebeing fixed to provide a constant temperature. As above indicated, thisconstant temperature is generally different from the initial position ofthe setpoint indicating member 10. This temperature droop" or offsetwill be naturally greater for a wide control band setting than a narrowcontrol band setting of the system. Such temperature offset can bereadily corrected by manually adjusting the setpoint control member tocompensate for this temperature offset."

The measuring unit 2 may be provided with a pair of colored lighttransmitting panels and 17 (FIG. 3) behind which lamps may be mounted.When the heater coil 12 is not receiving any current, one of the lightpanels 15 which may be a green colored glass will be lighted and whenthe heater coil 12 is receiving current the other light panel 17 whichmay be colored red will be lighted.

Refer now to FIG. 6 which indicates the most advantageous form of theinvention for controlling in a simple and reliable manner the width ofthe control band along with the rate of control. The output of the solarcell 18 in FIG. 6 represents a signal input component ei fed to anon-off signal level responsive circuit 26. In addition to the signalinput component er, the resultant voltage Er fed to the circuit 26includes a second signal input component ei' which is a voltagedeveloped across a capacitor 32 shown connected in series with theoutput of the solar cell 18.

The on-off signal level responsive circuit 26 controls the energizationof a relay 28 having a series of movable contacts 28-1, 28-2, and 28-3which make contact with stationary off" contacts 28-1, 28-2 and 28-3'when the relay 28 is deenergized. When the resultant input voltage Ercoupled to the on-off signal level responsive circuit 26 reaches avoltage level VI (e.g. 1 10 millivolts), relay 28 will become energized.The circuit 26 will continue to keep relay 28 energized until theresultant input voltage Er thereto changes direction and reaches avoltage level V2 (e.g. I00 millivolts) when the relay 28 becomesdeenergized. The heater coil 12 is shown connected between the movablecontact 28-] and an AC voltage source 29 to complete an energizationcircuit to the heater coil 12 when grounded stationary contact 28-1" isengaged by the movable contact 28-] when relay 28 is energized.

A capacitor charge circuit 34 generally indicated by reference numeral34 is provided which, for any given adjustment of the bandwidth and ratecontrol member 13, provides a different pair of DC capacitor chargingvoltages at the output of voltage sources 36 and 38. For sake ofillustration, it will be assumed that for the widest control bandwidth,the solar cell 18 will provide a 250 millivolt output when the solarcell 18 is unobstructed, that is for environment temperatures below thelowest limit of the control band involved, and will provide an output of40 millivolts when the solar cell is substantially completely obstructedby the masking element 24 due to ambient light conditions or otherreasons. In such case,

it is apparent that for the resultant input voltage Er to the onoffsignal level responsive circuit 26 to reach a level V1, 1 l0 millivolts,to operate the circuit 26 to an on condition, where the relay 28 isenergized when the environmental temperature is at the lowest end of thecontrol band, the signal input component ei' developed across thecapacitor 32 must be minus 140 millivolts (i.e. 250-I40 millivoltsequals 110 millivolts), and for the resultant input voltage Er to reacha level V2, 100 millivolts, to operate the circuit 26 to an offcondition, where relay 28 is deenergized when the environmentaltemperature is at the lowest end of the control band, the voltage er"developed across the capacitor 32 must be minus 150 mv. (i.e. 250-150millivolts equals I00 millivolts). For best control, the differencebetween the "on and "off voltage levels V1 and V2 is selected to beafraction of the difference in the DC output voltages of the DC voltagesources 36 and 38.

When the environmental temperature is at the upper end of the widestcontrol band involved, it is assumed that the output of the solar cell18 will be 40 millivolts. Under these circumstances, for the resultantinput voltage Er to the on-off signal responsive circuit 26 to reach l10 millivolts to effect energization of the relay 28, the voltage acrossthe capacitor 32 must be plus 70 mv. (i.e. 40+70 millivolts equals 100millivolts), and for the resultant voltage Er to reach 100 millivolts toeffect deenergization of the relay 28, the voltage across the capacitor32 must drop to plus 60 millivolts. For reasons to be explained, toprovide the proper signals to the on-off signal responsive circuit 26 tooperate the system as illustrated in FIG. 4, the magnitude V3 of theoutput voltage of one of the voltage sources 36 is selected so it isapproximately equal to the voltage (e.g. -l millivolts) to which thecapacitor 32 must be charged to operate the on-off signal responsivecircuit 26 to its off state when the environmental temperature is at thelow end of the control band. Also, the magnitude V4 of the output of theother voltage source 38 is selected so that it is at or near the voltage(e.g. +70 millivolts) to which the capacitor 32 must be charged tooperate the on-off signal of the responsive circuit 26 to its oncondition when the environmental temperature is at the upper end of thecontrol band involved.

The voltage sources 36 and 38 are selectively connected to the capacitor32 depending upon whether the relay 28 is energized or deenergized. Tothis end, as illustrated, the relatively negative terminal of the plus70 millivolt voltage source 38 and the relatively positive terminal ofthe negative millivolt voltage source 36 are connected by a conductor 40to the plate of capacitor 32 remote from the circuit 26, and therelatively positive terminal of voltage source 38 and the relativelynegative terminal of the voltage source 36 are respectively connected tostationary contacts 28-2 and 28-2" associated with the movable contact28-2 of the relay 28. When the relay 28 is energized to effect feedingof a heating current to the heater coil 12, the movable contact 28-2will make connection with the stationary contact 28-2" to connect thenegative terminal of the voltage source 36 to a capacitor chargingresistor 42 in turn connected to the plate of the I capacitor 32 nearestthe circuit 26. Thus, when the relay 28 is energized to effect a heatingoperation, the voltage source 36 is connected to the capacitor 32 tocause it to charge through resistor 42 to a voltage which causes thecircuit 26 to deenergize the relay. Similarly, when relay 28 becomesdeenergized, the movable contact 28-2 will make contact with the contact28-2 to connect the voltage source 38 to the capacitor 32 to cause it tocharge through resistor 42 to a voltage which causes the circuit 26 toenergize the relay.

The manner in which the capacitor 32 charges to the two voltagesinvolved to effect the operation of the proportioning control systemshown in FIG. 4 is illustrated in FIGS. 7A, 7B and 7C to which referenceshould now be made. FIG. 7A illustrates the condition of theproportioning control system when the environmental temperature is atthe lower end of the control band shown in FIG. 4, when the capacitor isconnected to the plus 70 millivolt voltage source 38 during the offcondition of the circuit 26 when the capacitor charges from minus 150millivolts to plus 70 millivolts. Since, in the example of the inventionbeing described, the voltage of the capacitor 32 never varies more thanmillivolts, in driving the circuit 26 to its on condition the voltageacross capacitor 32 will arrive very quickly to minus 140 millivolts toterminate the off condition of the circuit 26 because the capacitor ischarging to a voltage substantially different (i.e. plus 70 mv.) fromits'initially minus 150 millivolt condition. As illustrated in FIG. 8,when a capacitor charges to voltages V, W2 and W4 respectively ofdiminishing magnitude, the charging waveforms C2, C3 and C4 are ofprogressively diminishing steepness. Thus, when the circuit 26 is in itson condition and the minus l50 millivolt voltage source 36 is thenconnected to the capacitor 32, the capacitor charges very slowly tominus 150 millivolts because the capacitor is then charging to a voltagewhich is near its initially charged condition so it takes a relativelylong time for the capacitor to reach the minus 150 millivolt condition.This means that circuit 26 will have a relatively long "on" conditionand a relatively short off" condition, which is the desirable conditionfor the situation where the environmental temperature is near the lowend of the control band.

Referring now to FIG. 7C which illustrates the condition when theenvironmental temperature is near the upper end of the control band, itwill be seen that the capacitor will charge and discharge between plus60 and 70 millivolts to operate the circuit 26 between its off and on"conditions, and here the capacitor charges slowly from plus 60 to 70millivolts during the off condition of the circuit 26 since its initialvoltage of 60 millivolts is close to the 70 millivolts to which it ischarging, so it will take a relatively long time for the capacitor 32 toswitch the circuit 26 from its "ofi' to its on condition and so thecircuit 26 operates for relatively long intervals in its off" condition.When the capacitor 32 is discharging from plus 70 millivolts to plus 60millivolts during the on" condition of the circuit 26, the capacitorconversely reaches 60 millivolts rapidly since the capacitor is thencharging toward minus 150 millivolts, a voltage substantially differentfrom its initial voltage. Therefore, the circuit 26 will be operated inits on" condition for very short intervals.

H6. 73 illustrates the voltage waveform across the capacitor 32 when theenvironmental temperature is at the midpoint of the control band. Asindicated in FIG. 7B, the capacitor 32 will then charge between minus 30and minus 40 millivolts which is roughly midway between the voltagelevels V3 and V4 (minus 150 millivolts and plus 70 millivolts), socapacitor 32 charges and discharges between minus 30 and minus 40millivolts at approximately the same modest rates, resulting in roughlyequal on and off times of the circuit. Also, the period betweensuccessive cycles of the control signals is small (i.e. theproportioning rate is relatively high) in comparison to the periodrequired for each control cycle when the environmental temperature is atthe extremes of the control band involved.

Thus, it is apparent that when the capacitor 32 charges and dischargesbetween voltages the difference of which is a small fraction of thevoltage levels V3 and V4 between which the capacitor charge curvesextend, when the capacitor voltages are near one of the voltage levelsV3 (i.e. minus 150 millivolts in the example described), the capacitorhas a relatively small discharge (or charge) period during which thecircuit 26 is in its off" condition and a relatively long charge (ordischarge) interval during which the circuit 26 is operated in its foncondition. Similarly, when the voltages on the capacitor are near theother voltage levels V4, the capacitor 32 has a relatively shortdischarge (or charge) period during which the circuit 26 is operated inits on" condition and a relatively long charge (or discharge) periodduring which circuit 26 will be operated in its off condition.

FIGS. 9A, 9B and 9C represent the voltage waveforms across the capacitor32 when the proportioning control system is adjusted to provide thenarrowest bandwidth operation shown in FIG. 5. To accomplish thisresult, it is necessary to vary at least one of the voltage sources 36and 38 so that it has a voltage much closer to the voltage of the othervoltage source. For example, if the voltage source 36 changed from minusmillivolts to plus 36 millivolts, the control bandwidth is reduced tothe point where the lower temperature limit in the band is thattemperature where the solar cell output (reduced from its previousvalue) for such temperature plus the 36 millivolts equals the 100millivolts needed to turn the circuit 26 to its off" condition. Thus, inthe exemplary form of the invention being described, the solar cell 18will produce a voltage of 64 millivolts when the temperature indicatingpointer 6 is at 336 degrees Fahrenheit. The adjustment of the voltagesource 36 to such a voltage of 36 millivolts will then automaticallyreduce the control rate because the capacitor 32 will be charging towardvoltages in the extreme which are much closer to the voltages on thecapacitor to start with so it will take the capacitor 32 a much longertime to reach the voltage limits thereon involved.

FIG. 6 also illustrates the energization circuit for a green lamp 15'and a red lamp 17 which are respectively mounted behind the light panels15 and 17 of the measuring unit 2 (see FIG. 3). Accordingly, the greenlamp 15' has a terminal connected to the stationary contact 28-3' whichis engaged by the movable contact 28-3 when the relay 28 is deenergized.The red lamp 17 has a terminal extending to the stationary contact 28-3"engaged by the movable contact 28-3 when the relay 28 is energized. Theother terminals of the lamps l5 and 17 are connected to the source of ACvoltage 29. Also illustrated in FlG. 6 is that the galvanometer movement22 and the thermocouple 11 are associated with a bridge circuit 23 whichoperates the galvanometer movement 22 so the temperature indicatingpointer indicates the proper temperature on the measuring unit scale 4.

Refer now more particularly to FIG. 10 which shows the preferredcircuitry for the box diagram of FIG. 6. As shown in FIG. 10, thecapacitor charge circuit 34 is a bridge circuit which supplies thevoltages V3 and V4 to the capacitor 32 in accordance with theenergization or deenergization of the relay 28 controlling the flow ofpower to the heater coil 12. The DC input to the bridge circuit is atterminals U and V, terminal U being positive relative to terminal V.Extending in parallel between the input terminals U and V are bridgecir' cuit-forming branches 34a and 34b. Bridge circuit-forming branch34a as illustrated includes in series circuit relationship resistors 50and 52 and a parallel connection of a resistor 54 and a potentiometer56. The bridge circuit-forming branch 34b includes a resistor 58 inseries with a potentiometer 60 and a resistor 62. The output of thebridge circuit is taken between wiper 60a of the potentiometer and themovable contact 28-2 associated with bridge circuit-forming branch 34a.The wiper 56a extends to the aforementioned stationary on" contact 28-2"contacted by the movable contact 28-2 of the relay 28 when relay 28 isenergized. The juncture between resistors 50 and 52 in the bridgecircuit branch 34a extends to the stationary off" contact 28-2' contactby the movable contact 28-2 of the relay 28 when the relay 28 isdeenergized. lt should thus be apparent that when the movable contact28-2 engages the stationary off contact 28-2 the movable contact 28-2has a relatively high positive potential relative to bridge inputterminal V and when the movable contact 28-2 engages the stationary oncontact 28-2" the voltage on the movable contact 28-2 relative toterminal V will vary between zero and a small positive voltage as thewiper moves between the bottom and upper ends of the potentiometer 56.

The movable contact 28-2 is connected through the previously mentionedcapacitor charging resistor 42 to the terminal of the capacitor 32nearest the input to the on-off signal level responsive circuit 26. Thewiper 60a of the potentiometer 60 in the bridge branch 34b is connectedto the opposite terminal of the capacitor 32. The relative values of theresistors in th e bridge circuit branch 34a are so selected that whenthe movable contact 28-2 engages the stationary off contact 28-2',

the movable contact 28-2 will be positive with respect to the wiper 60aof the potentiometer 60 when the relay 28 is deenergized. The capacitor32 and resistor 42 are thus connected between the output of thecapacitor charging bridge circuit, namely between movable contact 28-2and wiper 60a. When the wiper 56a of the potentiometer .56 is at the topof the potentiometer as viewed in FIG. and the relay 28 is energized sothe movable contact 28-2 engages the stationary on contact 28-2", asindicated the voltage of the movable contact 28-2 will be plus 36millivolts with respect to the volt- ,age on the wiper 60a. Thisrepresents the narrowest bandwidth condition of the proportioningcontrol apparatus illus trated in FIG. 10. When the wiper 56a is at thebottom end of the potentiometer 56 when the relay is energized so themovable contact 28-2 engages the stationary on contact 28-2", thevoltage of the movable contact 28-2 will be minus 150 mvs. With respectto the voltage on the wiper 60a. This represents the widest bandwidthcondition of the proportioning control apparatus illustrated.

The capacitor 32 shown in FIG. 10 is coupled through a resistor 65 toone of the terminals of a capacitor 67 whose opposite terminal isconnected to a grounded line 70. The time constant of the charge circuitof the capacitor 67 is such that the capacitor 67 will have a voltagedeveloped there across which follows the sum of the voltage developedacross the capacitor 32 and the output of the solar cell 18 (or thecircuit thereof which, as illustrated, includes a series resistor 66 anda shunt resistor 68). The output of the capacitor 67 is fed to a more orless conventional shunt type chopper circuit generally indicated byreference numeral 69. The chopper circuit includes a transistor 71 whichis alternately rendered conductive and nonconductive by the connectionof its base electrode 71b through a resistor 73 and'a rectifier 75 tothe secondary winding 77a of a transformer 77 whose primary winding 77bis connected to a source 78 of 60 cycles per second AC voltage.Rectifier 75 couples half-wave rectified pulses from the transformersecondary winding 77a so the input to the chopper circuit 69 iseffectively chopped at a 60 cycle per second rate where it can bereadily amplified in an amplifier indicated by reference numeral 76. Theoutput of the chopper circuit 69 therefore comprises a series ofpositive pulses P4 the amplitude of which corresponds with theincreasing and decreasing portion of a waveform representing the sum ofthe output of the solar cell 18 and the capacitor 32.

The output of the chopper circuit 69 is fed to the base electrode 79b ofa NPN transistor 79. This transistor forms with the associated circuitelements a class A amplifier having negative going pulses P4 in theoutput thereof taken at the collector electrode 79c which is coupledthrough a capacitor 81 to the input to a Schmidt trigger circuit 83. TheSchmidt trigger circuit shown in FIG. 10 trigger a unique Schmidttrigger circuit which, unlike conventional Schmidt trigger circuits, hasat least one of the portions thereof energized from a rectified sourceof alternating current to produce a pulsating energizing currenttherefor.

The Schmidt trigger circuit comprises a pair of NPN transistors 85 and87 having respective emitter electrodes 85c and 87e connected through acommon feedback resistor 89 to the ground line 70. The collectorelectrode 85c of the transistor 85 is coupled through a resistor 90 to apositive bus 92 which also feeds a biasing network 91 for the transistor85 which normally biases the transistor to a fully conductive state. Thebase electrode 85b of transistor 85 is connected to the capacitor 81.The resultant flow of current through the common feedback resistor 89will normally develop a sufficiently high positive voltage to keep thetransistor 87 relatively nonconductive. When the input voltage fed tothe chopper circuit 69 reaches 1 l0 millivolts, the resultant negativegoing portion of the chopped signal pulses P4 developed at the output ofthe transistor 79 will drop the voltage at the base electrode 85b to alevel which will drive the transistor 85 into a nonconductive statewhich, if the other transistor 87 is able to conduct, will formtherewith a Schmidt trigger circuit wherein, through the feedback actionof resistor 89 and a resistor 94 connected between the collectorelectrode 850 of transistor and the base electrode 87b of transistor 87,reverses the conductive states of transistors 85 and 87 almostinstantaneously so a sharp pulse P5 appears at the collector 850 oftransistor 85. This pulse persists until the instantaneous value of thesignal pulse P4 involved drops to a lower level than that whichtriggered the circuit. This represents the usual hysteresis action of aSchmidt trigger circuit. The relay 28 and/or the circuit controlling thesame to be described has a delay which will not result in theenergization of the relay 28 until a large number of pulses appear onthe collector electrode 85c so that the negative going pulses P4 fed tothe transistor 85 will build up in amplitude after the first pulse P4triggers the transistor 85 into its conductive state until relay 28operates. Transistor 85 is repeatedly driven into conduction by eachpulse P4 during the on condition of operation of the signal levelresponsive circuit 26 until the input signal from the capacitor chargecircuit 34 drops the resultant voltage Er to millivolts in a manner tobe explained.

As is well known, a Schmidt trigger circuit has a substantial hysteresischaracteristic so that a voltage which triggers the same into one stateof conduction must reverse direction and reach a substantially differentlevel before the trigger circuit reverses its conductive state. Thetransistor 87 which is normally in a relatively nonconductive state, isenergized by positive half-wave rectified 60 cycle per second voltagepulses P6 rather than a constant amplitude DC voltage as in the case ofconventional Schmidt trigger circuits. To this end, the collectorelectrode 870 of the transistor 87 is coupled through a resistor 103 anda rectifier 106 to a line 108 extending to a center-tapped secondarywinding 77c of the transformer 77. The phase of the positive half-waverectified pulses P6 fed to the transistor 87 is the same as thehalf-wave rectified pulses P4 fed to the base of the chopper circuittransistor 71 so out of phase repetitive or cyclic interference pulseswill not operate the Schmidt trigger circuit. After the initialtriggering of the Schmidt trigger circuit in the manner described above,the amplitude of the negative going pulses fed by the capacitor 81 tothe base electrode 85b of the transistor 85 is reduced in amplitude to alevel caused by the drop in the input voltage to the circuit 26 to theaforementioned value of 100 millivolts. The point is reached where thebias provided on the base electrode 85b by the biasing network 91 onceagain establishes a voltage condition which causes the transistor 85 tobecome continuously relatively highly conductive again (the choppersignals will cause the switching off of the Schmidt trigger circuitbetween the chopper pulses even when such pulses trigger the circuit).

The aforesaid resistor 94 forms with a resistor 96 coupled between thebase electrode 87b of the transistor 87 and the ground line 70 a voltagedivider. A filtering capacitor 98 is coupled between the collectorelectrode 85e of the transistor 85 and the ground line 70. The voltageon the capacitor 98 is coupled through the resistor 100 to the input ofa relay control circuit 102 including a transistor 104 normally in arelatively nonconductive state. When the voltage across the capacitor 98is above a given threshold level, the transistor 104 becomessufficiently conductive to energize the relay 28 connected in the loadcircuit of the transistor 104.

The base electrode 104b of a transistor 104 is connected by a resistor111 to a conductor 113 leading to the coupling resistor 100. A noisefiltering bypass capacitor 115 is coupled between the conductor 113 andthe ground line 70. A rectifier 117 is connected between the emitterelectrode 104e of the transistor 104 and the ground line 70. The relay28 is connected between the collector electrode l04c of the transistor104 and a point 120 to which is coupled a full wave rectified ACvoltage. Accordingly, a rectifier 122 is connected between the point 120and the aforementioned conductor 108 extending to one end of thecenter-tapped secondary winding 770. The point 120 is also coupled by arectifier 124 to a conductor 126 extending to the other end of thesecondary winding 77c.

A reverse voltage preventing rectifier 127 to protect transitor 104 fromhigh voltages induced in the relay 28 is connected in parallel with therelay when the voltage input to the chopper circuit 69 rises to 110millivolts in the example of the invention being described, the voltageon the capacitor 115 is suffi ciently high to raise the current flow intransistor 104 to a degree where the magnitude of the current flowingthrough the relay 28 will energize the same.

To provide a highly stabilized overall circuit hysteresis which willeffect turnoff of the circuit 26 at the 100 millivolt input described,combined with the very small hysteresis Schmidt trigger circuit is apositive feedback brand extending between the input point to the relaycontrol amplifier 102 at the ungrounded end of a grounded capacitor 115connected through resistor 111 and conductor 113 to resistor 100 and theinput to the chopper circuit 69. The feedback branch includes arelatively large resistor 116 connected to the ungrounded plate of thecapacitor 67 at the input of the chopper circuit 69. This feedbackbranch, among other things, increases the positive voltage on thecapacitor 67 so that once the circuit 26 is triggered to its on"condition, an artificially increased negative going signal appearsacross the input of the Schmidt trigger circuit so that the relay 28does not become deenergized until the amplitude of the signal input fromthe capacitor charge circuit 34 and solar cell drops to the exemplary100 millivolt level in other words, this feedback branch provides anincreased difference in the voltage necessary to operate the on-offsignal responsive circuit 26 between its on" and off than would be thecase without this branch circuit. In effect, therefore, it is thefeedback branch which provides the differential signal levels whichoperates the Schmidt trigger circuit between its on" and "off'conditions (the "on condition thereof meaning that the pulses P4 areeffective in triggering the transistor85 of the Schmidt trigger circuiteach cycle to a relatively nonconductive state). In effect, during thealternate half-cycles when transistor 87 is not receiving any energizingvoltage, transistor 85 and the associated rectifier elements act like anordinary amplifier and, when transistor 87 receives the energizingpulses P6, the transistors 85 and 87 act as a Schmidt trigger circuit.The pulsing of transistor 87 results in a greatly reduced hysteresis forthe trigger circuit and enables the circuit to act as a phase detector.

The power source 30 may be a more or less conventional power source. Asillustrated, it includes a full wave rectifier circuit 150 comprisingrectifiers 152 and 154 coupled between the opposite ends of theaforementioned secondary windings 77c. The anode electrodes of therectifiers 152 and 154 are coupled to a common conductor 156 leading toone of the plates of a filter capacitor 158 whose opposite plate isconnected to a conductor 160 extending to the center tap point of thesecondary winding 770. The conductor 166 extending to the center tap ofthe transformer 77c acts as a source of positive potential and isconnected to the aforementioned positive bus 92 through a conductor 162.The ground line 70 is coupled by a conductor 164 to the emitterelectrode l66e of a transistor 166 whose collector electrode 166a isconnected to one end'of a resistor 168 whose opposite end is connectedto the conductor 156 constituting the negative output of the full waverectifier circuit 150. The base electrode l66b of the transistor 166 isconnected to Zener diode 170. A resistor 172 is connected from thejuncture of the Zener Diode 170 and the base electrode 166b oftransistor 166 and the juncture between capacitor 158 and the resistor168 to provide a stable reference voltage for the well-known seriesregulator circuit formed by transistor 166.

The transformer 77 has a third secondary winding 77d one of the ends ofwhich is coupled through a rectifier 175 to one of the plates of afilter capacitor 177 whose opposite plate is connected to the other endof the secondary winding 77d. The capacitor 177 is connected through aresistor 180 across the terminals of a Zener diode 182 which stabilizesthe voltages across the aforementioned terminals U and V which feed theinput to the circuit 34 forming a charge circuit for the capacitor 32.

The positive conductor of the power supply is connected by a conductor184 to the positive energizing voltage input terminal 186 of themeasuring unit bridge circuit 23 which operates the aforementionedgalvanometer movement 22. The other energizing input terminal of thebridge circuit 23 is at a grounded wiper 188a of a potentiometer 188.One end of the potentiometer 188 is connected through the thermocouple11 to the positive input terminal 186 of the bridge circuit 23 and theother end of the potentiometer 188 is connected to a series circuitcomprising resistors 190 and 192 and the parallel circuit comprising theresistor 194 and the potentiometer 196 leading to the positive inputterminal 186. The wiper 196a of the potentiometer 196 is arranged toshort circuit progressively increasing portions of the potentiometer 196to act as a bridge adjusting or calibrating means. Potentiometers 196and 188 are arranged coaxially and therefore adjusted simultaneouslywith one adjustment.

The galvanometer movement 22 is connected across the output terminals200 and 202 of the bridge which are respectively located at the juncturebetween resistors 190 and 192, on the one hand, and potentiometer 188and thermocouple 11 on the other hand. The galvanometer movement isshown connected in series with a resistor 204 in parallel with a coppermetal coil temperature compensating resistor 206 and a resistor 208.Cold junction temperature is compensated by a bimetal attached to theupper hairspring of the meter movement and responsive to ambienttemperature changes. Potentiometer 188 balances out thermocouple leadresistance while providing break protection in the event thethermocouple opens.

Refer now to FIG. 11 which shows the most preferred form of the.invention. One of the differences in this circuit from that shown inFIG. 10 is the design of the capacitor charge circuit generallyidentified by reference numeral 34'. The circuit 34' is designed in sucha way that an automatic offset eliminating means can be simply addedthereto, as shown in FIG. 14 later to be described. Capacitor chargecircuit 34 is a voltage divider circuit which provides a proportioningsignal across a pair of terminals F and B.

The voltage divider circuit has voltage input terminals 211 and 213across which a source 212 of DC voltage is connected. Terminal 211 isshown connected to the positive terminal of the voltage source 212 andterminal 213 is shown connected to the grounded negative terminalthereof. Resistors 216, 217 and 219 are respectively connected in seriesbetween the voltage input terminals 211 and 213. In the most preferredform of this invention, the middle resistor 217 is in variableresistance of relatively small magnitude in comparison to resistors 216and 219. For example, resistor 216 may be 2,000 ohms, resistor 217 maybe variable between l0 and 200 ohms and resistor 219 may be 350 ohms.(It should be understood that the specific and relative values of theresistor may be varied widely without deviating from the broader aspectsof the invention). A pair of resistors 221 and 224 are connected inseries across the variable resistor 217. Exemplary resistance values forresistors 221 and 224 are 221,000 ohms. The aforementioned terminal F isconnected to the juncture of resistors 221 and 224 through a relativelylarge resistor 226, which may be, for example, I to 2 megohms. Theterminal B may be connected to the juncture between the resistors 221and 224 through a relatively large resistor 228 which may have a value,for example, of 0.7 megohms. A capacitor 32', which serves a similarfunction to the capacitor 32 in FIG. 2 is connected to the terminal Bthrough a relatively large resistor 230, which, for example, may be 1megohm. The capacitor 32', which may be 500 microfarads, is connected tothe movable contact 28-2 of the heating or cooling signal control relay28 through a relatively small resistor 232 which is insignificant insize relative to. the aforementioned resistors 228 and 230. For example,the resistor 232 may have a value of 68,000 ohms. The movable contact28-2 engages stationary foff contact 28-2 when the relay 28 isdeenergized and engages stationary. on" contact 28-2" when relay 28 isenergized. The "off contact 28-2 is connected to the upper end of thevariable resistor 217 of the voltage divider network and the on"stationary contact 28-2" is connected to the bottom end of the variableresistor 217.

FIG. 12A shows the circuit configuration of the capacitor charge circuit34' when movable contact 28-2 engages the on contact 28-2 and FIG. 13Ashows the circuit configurationof the capacitor charge circuit 34' whenthe movable contact 28-2 engages the off contact 28-2". It can be seenfrom the relative resistance values shown in FIGS. 12A and 13A that indetermining the simplified charge circuits for the capacitor 32 (FIGS.12B and 138) when the relay 28 is respectively energized and deenergizedresistors 230 and 228 can be ignored. Thus, as shown in FIG. 12B,capacitor 32' effectively charges toward the voltage E1 at the bottom ofthe variable resistor 217 through the relatively small resistor 232 whenthe relay 28 is energized and, as shown in FIG. 13B, capacitor 32charges toward the voltage IE2 at the top of the variable resistor 217when relay 28 is deenergized. The voltage on the terminal B with respectto ground and across the resistor 228 (to the opposite ends of whichterminals B and F are coupled) varies with the charge and discharge ofcapacitor 32'. The voltage on terminal F with respect to ground isessentially the voltage at the juncture between the resistors 221 and224. It is apparent from FIG. 12A that during the on" mode of operation,the capacitor 32 will be discharging from a relatively higher voltage toa lower voltage and during the off" mode of operation the capacitor 32'will be charging from an initially lower voltage to a relatively highervoltage. The waveforms of the voltage across the capacitor 32 will besimilar to that shown in FIG. 7A-C and 9A-C and the voltage on terminalB with respect to ground will vary accordingly. Thus, the voltage acrossthe capacitor 32' will vary between two voltage levels which have aconstant difference which produces the difference in signals necessaryto operate the associated on-off differential signal level responsivecircuit which is indicated in FIG. 11 by reference numeral 26. Theproportion of the on" and off intervals of the proportional controlsignal will depend upon the same factors described in connection withthe circuit of FIG. 10.

The control bandwidth as well as the control rate are varied by varyingthe magnitude of the resistor 217 since-this varies the relative valuesof the voltages E1 and E2. Also, in this form of the invention as wellas the form of the invention shown in FIG. variation in the control ratecan be obtained by changing the value of the resistor through which thecapacitors 32 and 32 involved charge.

It is truly surprising to find that an automatic offset eliminatingcircuit can be obtained, as shown in FIG. 14, by the simple addition tothe circuit 34' of a capacitor 32" connected between ground, a resistor226' which is a very large resistor, such as a 2 megohm resistor,connected to the movable contact 28-2 and a resistor 236' connected topoint F. The time constant afforded by the capacitor 32 and resistor 232is extremely long relative to the time constant of the capacitor 32' andthe resistor 232, so that the full effect of the capacitor 32" does notappear for a relatively long time, such as 25 minutes. (The value ofcapacitor 32 in one exemplary form of the invention was 220microfarads). The voltage across the capacitor 32 is coupled to theterminal F through a relatively large resistor 230 which may be Imegohm. It is apparent that when the capacitor 32" finally charges tothe value of the capacitor 32', there will be no voltage differencebetween terminals B and F. Although the voltage across the capacitor 32or terminals F and B could be added in series with the output of a solarcell circuit in a manner similar to that described in connection withthe embodiment of FIG. 10, terminals F and B are preferably connected toa very unique combination bridge and chopper circuit 23 which carriesout the functions of the measuring circuit 23, the solar cell circuitand the input chopper circuit 69 in the circuit of FIG. 10 using thesame grounded DC voltage source 212. (Note that the bridge circuits 23and 34 operate with a floating ground which reduces common modeproblems). The circuit 23 has a general utility in measuring circuitsgenerally, although it also has a particular utility in the proportionalcontrol circuit of the invention. The bridge portion of the circuit 23in the absence of any connection to the capacitor charge circuit34' willproduce across a pair of bridge output terminals 213 and 215 a signalwhere the terminal 213 will be negative with respect to the terminal 215to a decreasing degree as the set point temperature is approached. Theoutput of the bridge portion of the circuit will be zero at the setpoint temperature so the voltage of terminal 213 relative to theterminal 215 will be increasingly positive for temperatures above theset point temperature.

To explain the operation of the invention, it is more convenient tofollow the voltage variation between the output terminals 213 and 215with reference to ground appearing on a ground line 217 constituting oneof the energizing voltage inputs of the bridge portion of the circuit23. The other energizing voltage input terminal is terminal 219connected to the source 212 of DC voltage. The bridge portion of thecircuit includes a branch 23a including a resistor 231 connected betweena positive bus 223 and one end of a potentiometer 235 whose opposite endis connected in series with a resistor 237 and a cold junctiontemperature compensating resistor 239 connected to the ground line 217.A thermocouple 11' is connected between the wiper 235a of thepotentiometer 235 and the aforementioned bridge output terminal 213. Thethermocouple 11 provides a progressively increasing voltage withincreasing temperature, with the right-hand end thereof being positivewith respect to the left-hand end so that the voltage at the bridgeoutput terminal 213 will progressively become more positive withincrease in temperature and will reach a positive voltage which will besubstantially the same as the positive voltage on the bridge outputterminal 215 at the set point temperature (assuming no connections aremade to the capacitor charge circuit 34). A resistor 244 is shownconnected between the output terminal 213 and the positive bus 223. Theresistor 244 is an extremely large resistor relative to resistor 231 andis utilized to provide continuity for the bridge output circuit shouldthe thermocouple l1 become open circulated. For all practical purposesthe resistor 244 can be ignored.

Unlike the bridge circuit 23 in FIG. 10 where the circuit operates inthe millivolt range and the thermocouple resistance affects themeasurement of the circuit, variations in thermocouple lead length havelittle effect on the circuit because of the position of the thermocouplein the circuit and the fact that the associated resistors are so muchlarger than the thermocouple lead length resistance.

The voltage on the bridge output terminal 213 is coupled to a filternetwork comprising a resistor 243 connected between the terminal 213 andthe output terminal of a capacitor 234 whose opposite terminal isconnected to the ground line 217. The resistor 243 and the capacitor 234act as a noise filter for noise signals picked up by thermocouple leads.The ungrounded terminal of the capacitor 234 is coupled through aresistor 248 (which, for example, may be a 4700 ohm resistance) to theupper terminal of a capacitor 250 (which may have a value, for example,of 20 mfd.) whose opposite terminal is connected to'the ground line 217.The aforementioned terminal F of the capacitor charge circuit 34' isconnected to the ungrounded terminal of the capacitor 250. In effect,the voltage (or charge resulting therefrom) on the capacitor 234 and thevoltage on the terminal F of the capacitor charge circuit 34' withrespect to ground are mixed or effectively added in the capacitor 250.As previously indicated, in the form of the invention shown in FIG. 11,the voltage of the terminal F does not vary significantly with referenceto ground.

The bridge portion of the circuit 23 has another branch 23b extendingbetween the DC voltage input terminals 219 and the ground line 217. Thisbranch 23b as illustrated includes a resistor 231' connected between thepositive bus 223 and a variable resistor 233 which in turn is connectedto a parallel circuit comprising a potentiometer 235' and a resistor237. The wiper 235a ofpotentiometer 235 is connected to the bridgeoutput terminal 215. The latter parallel connected resistors areconnected in series with a resistor 239' connected in parallel with apotentiometer 241. The latter resistors are connected through resistor243' to the ground line 217.

A suitable temperature indicator 245 may be connected between theaforementioned bridge output terminal 213 and the wiper 241a of thepotentiometer 241. The points in the bridge circuit to which indicator245 are connected are not effected by the proportioning control voltagefed from the capacitor charge circuit 34'.

The other bridge output terminal 215 is connected through a resistor 248corresponding in value to the aforementioned resistor 248 to the upperterminal of a capacitor 250' corresponding in value and function to thecapacitor 250. The terminal B of the capacitor charge circuit 34 isconnected to the ungrounded terminal of the capacitor 250. Thus, the DCvoltage on the bridge output terminal 215 or charge resulting therefromis added or mixed with the voltage at the terminal B of the capacitorcharge circuit 34 or the charge resulting therefrom in the capacitor250'.

When the environmental temperature is at the lower end of the controlband involved, the voltage fed to the terminal B from the capacitorcharge circuit 34' will act in a direction to make the voltage of thecapacitor 250' less positive with respect to the voltage across thecapacitor 250 so that if one looked at the output of the bridge circuitby comparing the voltages at the ungrounded terminals of the capacitor250' and 250 one would see a greater voltage difference than wouldotherwise be the case. Thus, considering the voltages across thecapacitor 250 and 250' to be the output of the bridge circuit, thebridge would be balanced at a lower temperature than would otherwise bethe case.

The chopper portion of the circuit 23' generally indicated by referencenumeral 255 alternately and cyclically connects sistances substantiallyunder 1,000 ohms in their conductive a capacitor 257 across thecapacitors 250 and 250'. The time constant of the charge circuits forthe capacitor 257 is sufficiently short that it will completely chargeup to the voltage on the capacitors 250 and 250 during the time it isconnected separately to these capacitors. The voltage on the capacitor257 will, therefore, change only as the relative voltages stored in thecapacitors 250 and 250' change. A conductor 259 connects the ungroundedterminal of the capacitor 257 to a DC blocking capacitor 261 which inturn feeds the input to the onoff differential signal level responsivecircuit 26' whose input signal therefore is a function solely of thedifference in the voltages across the capacitors 250 and 250'. Thisvoltage difference will advantageously fluctuate in the microvolt range,such as over a range of I00 microvolts more or less in the exemplaryform of the invention.

The on-off differential signal level responsive circuit 26 used with thecircuit 23 being described will be assumed to be in an off conditionwhen the voltage fed thereto is zero and in an on condition when thevoltage fed thereto is, say, for example, 100 microvolts. (If desired,any suitable "off condition offset can be provided for convenience indesign of the circuit 26'.)

Junction transistors do not usually operate as effective switches atmicrovolt levels because transistor switches generally have too highresistance values during conduction and produce too high voltage dropsto be operable in the microvolt range. Also, the voltage dropthereacross varies substantially with temperature. However, field effecttransistors have been found operable with microvolt level signals. Fieldeffect transistors act like linear resistors with such applied voltagesand have a 100 to l impedance ratio between conducting and nonconductingconditions. For example, field effect transistors commonly haveresistances in excess of 200,000 ohms in their nonconductive conditionand reconditions. Accordingly, as shown in FIG. 11, the ungrounded endof the capacitor 257 is connected to the drain electrodes D and D of apair of field effect transistors 263 and 263. The source electrodes Sand S of the field effect transistors 263 and 263' are connected to theungrounded terminals of capacitors 250 and 250. In the circuit beingdescribed, the resistors 265 and 265' may be, for example, 3300 ohms.The gate electrodes G and G of the field effect transistors 263 and 263'are respectively connected through resistors 265 and 265 to signal inputleads 267 and 267' which receive clipped half-wave rectified negativepulses P7 derived from the secondary winding 270a of transformer 270whose primary winding 27011 is connected to a suitable source 271 of 60cycle AC voltage. The opposite ends of the secondary'winding 270a areconnected respectively through rectifiers 272 and 272 and resistors 274and 274 to the signal input leads 267 and 267 respectively. Zener diodes277 and 277 connected between the terminals 267 and 267 and the groundline 217 act as clippers which clip the half-wave rectified voltage atsteep points in the voltage waveforms involved so that negative pulsesappearing as square waves are fed to the terminal 267 and 267. Resistors279 and 279 are connected in parallel with the Zener diodes 277 and277'. The negative voltage appearing on the leads 267 and 267' duringalternate half cycles of the 60.cycle waveform of the AC voltage source272 alternately drive the field effect transistors to a high resistancecondition so that during the intervening half cycles the field effecttransistors conduct to couple the capacitor 257 alternately through theresistors 265 and 265 to the capacitors 250 and 250'.

The combination bridge and chopper circuit 23' requires only a singlepower supply operating with a grounded input which reduces the cost,eliminates or minimizes noise and common mode problems and increases thereliability of the circuit.

The on-off differential signal level responsive circuit 26' operatessimilarly to the circuit 26 previously described and therefore adisclosure of the circuit 26 has not been made. It is apparent, however,that the input of the circuit 26'omits the chopper circuit 69 previouslydisclosed in the circuit 26. Also, the end of the feedback circuit usedin the circuit 26 to feed a DC back to the input capacitor 67 of thechopper circuit 69 is shown in FIG. 11 by a conductor 280 leading tocapacitor 250'.

In order to understand the theory of the automatic reset circuit shownin FIG. 14 let us approach it from the most simplified type of controland then onward in increasing complexity. FIG. 15a shows a simple on-offcontrol system with a defined 2 differential (set point 200, on" at 199and off at 201). FIG. 15b represents a proportioning or anticipatorycontrol system as shown in FIGS. 10 or 11. As capacitor 32 or 32'charges up, the resultant voltage would move the off point down to 180within, for example, 25 seconds after turn on" and before the processtemperature reaches the set point. As the process temperature wouldapproach 182, the controller would turn off. The already describedproportioning would then take place as a result of the voltage gradientinvolved. At some point there will be a harmonious relationship betweenthe pulses to the load and the sensor input information at which neitheryields any new error. In that case, the process temperature will settleout somewhere within the control band (depending on process-needs).Unless the process required a 50 percent heat input, the true ultimatetemperature could be anything but the set point within the control bandof from 180 to 220 F. If a greater percentage of heat than 50 percentwere required, the ultimate temperature would be less than 200 F. and,conversely, if a lesser percentage of heat were required the ultimatetemperature would be between 200 and 220 F. If this temperature would beat some discrete point and external factors (line, load, inertia, mass,etc.) change, a new offset temperature would be established. This meansthat even if the set point were manually changed to allow the finaltemperature to be 200 F changes in external factors could move thisaround over a period which may be intolerable.

4 Refer now to FIG. 15c which illustrates operation of the automaticreset circuit of FIG. 14 where the process temperature has not yetarrived near the control hand. For the first, for example, 25 seconds orsoafter turn on," the "ofF point is moved down to 182 F. During thenext, for example, 25 minutes, capacitor 32" charges to a voltage whichis roughly the same as that to which capacitor 32' charges which voltageis applied to point F by the chopper circuit in opposition to thevoltage applied to point B so the off point will move gradually back to201 (or that point where it would be as a simple onoff controller). Itcan be seen then that over a large period the average voltage value ofthe capacitors 32 and 32 are the same. Without automatic reset thesystem would provide an offset, for example, to 190 F. With automaticreset it might momentarily control there, but capacitor 32" wouldgradually charge to the average capacitor 32'. Since capacitor 32" willeventually charge to the average value of the voltage acrosscapacitor32' the control voltage fed to the circuit 26 ultimately benearly zero meaning that, except for instantaneous values appearingacross capacitor 32 causing proportioning action, the control will be atthe set point. During this longer reset period (due to capacitor 32")instantaneous on-off pulses providing proportioning will not havechanged materially. For this example we can assume that the load wantsto be on 75 percent of the time and off 25 percent of the time. Itfollows in usual processes that this ratio will not change much over apercent change in control temperature.

Now referring to FIG. 15d, let us assume that the process involved nowrequires a percent on" time to effect straight line control. As shown inFIG. 15d, some or so minutes after turn on", the control system willbegin to turn off first around 200 F. Since this process requires only a20 percent on time for proper control in this temperature area theactual process temperature will for the moment somewhere near 240 (let'ssay 235 F.). At this point capacitor 32" will begin its long timeconstant charge to approach the average voltage which is acrosscapacitor 32, the actual process temperature begins to move downscale atan exponential rate. At the time that the capacitors 32' and 32" have.the same average charge, the instantaneous waveform of the voltage oncapacitor 32' will effect a 20 percent on" time, but the averagevoltages from both capacitors will be cancelled out as explainedpreviously. FIG. l5e shows the final result of the automatic resetcurrent.

It is understood that more modifications may be made in the mostpreferred forms of the invention described previously without deviatingfrom the broader aspects of the invention.

We claim:

1. A proportional temperature control apparatus for generating at anoutput heating or cooling producing signals to be cyclicallyintermittently applied over a range of temperatures so the proportion ofeach cycle a heating or cooling producing signal is present is a maximumat one end of said temperature range and progressively decreases to aminimum near zero at the other end of the temperature range, saidapparatus comprising: manually operable temperature set point means forselecting the midpoint of the range of temperatures over which theproportional temperature control is to take place, temperature measuringmeans responsive to the temperature of the environment to be heated orcooled, a control circuit responsive to said manually operabletemperature set point means and said temperature measuring means forgenerating said cyclically applied signals at said output within theselected temperature range, said control circuit including bandwidth andrate control means for simultaneously adjusting'both the number ofdegrees included within said range of temperatures over which theproportional control is effected, and the rate at which said heating orcooling signals are cyclically applied to said output at correspondingpoints within the selected range of temperatures increasing with thenumber of degrees within the temperatures range over which proportionalcontrol is effected.

2. ln proportional temperature control apparatus including an output atwhich heating or cooling producing signals are to be cyclically appliedso the proportion of each cycle a heating or cooling producing signal ispresent is a maximum at one end of said temperature range andprogressively decreases to a minimum near zero at the other end of thetemperature range, manually operable temperature set point means forselecting the midpoint of the range of temperatures over which theproportional temperature control is to take place, and temperaturemeasuring means responsive to the temperature of the environment to beheated or cooled, the improvement comprising: a differential inputsignal responsive circuit which generates a heating or cooling producingsignal at the output of said apparatus when an input signal fed theretoreaches a first level and continues to generate said heating or coolingproducing signal at said Output until said input signal reversesdirection and reaches a second signal level, whereupon said heating andcooling producing signal disappears until the resultant input signalagain reaches said first level; first voltage generating meansresponsive to said temperature measuring means and said manuallyoperable temperature set point means for providing a first input voltagecomponent which progressively varies between first and second voltagelevels as the temperature of said environment varies between the limitsof said temperature range over which proportional control is to beeffected; second voltage generating means which provides a second inputvoltage component which is mixed or added to said first input voltagecomponent; signal feeding means for feeding a signal to said input ofsaid differential input signal responsive circuit which is a function ofthe added or mixed voltage components; said second voltage generatingmeans including a capacitor across which said second input voltagecomponent is developed, and a charge circuit for the capacitor includingmeans for selectively providing first and second DC capacitor chargingvoltages having values spaced apart a number of voltage unitssubstantially greater than the variation of said second voltage inputcomponent, and means responsive to the presence of heating or coolingsignals in the output of said temperature control apparatus for causingthe capacitor to charge toward one of said DC voltages to bring thevoltage across the capacitor to a value which provides a resultantsignal at said second level at the input of said differential inputsignal responsive circuit which causes the heating or cooling signals todisappear from said output, and responsive to the absence of heating orcooling signals in the output of said temperature control apparatus forcausing the capacitor to charge toward the other of said DC voltages tobring the voltage across the capacitor to a value which provides aresultant signal at first level at the input of said differential inputsignal responsive circuit to reestablish said heating or cooling signalsat the output of said temperature control apparatus.

3. The proportional temperature control apparatus of claim 2 wherein thetime constant of said capacitor charge circuit over the varioustemperatures involved within said temperature range remainssubstantially constant.

4. The proportional temperature control apparatus of claim 2 whereinsaid voltage charge circuit of said second voltage generating meansincluding means for selectively varying the value of at least one ofsaid first and second DC capacitor charging voltages.

5. The proportional temperature control apparatus of claim 2 whereinsaid means responsive to the presence or absence of the heating orcooling signals in the output of the apparatus includes a bridge circuithaving opposite branches connected across a pair of voltage energizinginput terminals, the capacitor and a charging resistance being connectedacross intermediate points of said branches constituting outputterminals of the bridge circuit, and means for selectively varying thepoints of connection of said capacitor and charging resistance to atleast one of said branches in accordance with the presence and absenceof the heating or cooling signals in the output of said apparatus toprovide said first and said second capacitor charging voltages.

6. The proportional control apparatus of claim wherein one of saidbranches has a potentiometer with a wiper constituting one of saidpoints of connection of said capacitor and charging resistance and theother point of connection thereof in said one branch being a point otherthan the potentiometer, wherein the variation of the position of thewiper on the potentiometer will progressively vary the magnitude of oneof said capacitors charging voltages.

7. The proportional control apparatus of claim 2 wherein said meansresponsive to the presence and absence of the heating or cooling signalsin the output of the apparatus includes a voltage divider circuit havingat least three series connected resistor means extending between a pairof DC energizing voltage input terminals, said capacitor having oneplate coupled to a point in the voltage divider circuit remote fromadjacent ones of said resistor means, and switch means respectivelycoupling the other plate of said capacitor through a charging resistormeans to the remote ends of said adjacent resistor means when theheating or cooling signals are respectively present and absent in theoutput of the proportional control apparatus.

8. The proportional temperature control apparatus of'claim 7 whereinsaid one plate of said capacitor is coupled to'the juncture of saidadjacent resistor means through at least two series connected couplingresistor means of very high value relative to said capacitor chargingresistor means so the latter is insignificant relative thereto, and saidmeans for feeding a signal to said differential input signal responsivecircuit including means responsive to the voltage across the one of saidseries connected coupling resistor means remote from said one plate ofsaid capacitor.

9. The proportional temperature control apparatus of claim 8 providedwith automatic ofiset eliminating means comprising a second capacitorand a second charging resistor means connected in series across thefirst-mentioned capacitor and its charging resistor means, the timeconstant of the second capacitor and its charging resistor means beingextremely long relative to the time constant of the first-mentionedcapacitor and its charging resistor means, and means for coupling thevoltage across said second capacitor tothe terminal of said one couplingresistor means remote from said first-mentioned capacitor, wherebyultimately the voltage across said one coupling resistor means dropssubstantially to zero.

10. The proportional temperature control apparatus of claim 8 whereinsaid first voltage generating means comprises a DC bridge circuit havinga pair of DC voltage input terminals and a pair of resistance-containingbranches extending between said voltage input terminals, there beingassociated with at least one of said branches variable responsive meanswhich produces a variation in the voltage between a point associatedwith said one branch constituting an output terminal of the bridgecircuit and a point in common between said branches, said manuallyoperable temperature set point means including means for varying thepoint of connection of a second output terminal for the bridge to saidother branch; second and third capacitors respectively coupled betweensaid bridge output terminals and said common point; and said means forfeeding a signal to the input of said differential input signalresponsive circuit includes means for coupling the output across saidone coupling resistor means of said voltage divider circuit respectivelyacross the terminals of said second and third capacitors remote fromsaid common point, said voltage divider and bridge circuits sharing acommon DC voltage source, a fourth capacitor having one plate connectedto said common point, switch means for connecting the other plate ofsaid fourth capacitor alternately and cyclically to the plate of saidsecond and third capacitors remote from said common point, and means forcoupling the output of said fourth capacitor to the input of saiddifferential input responsive circuit.

11. The proportional temperature control apparatus of claim 2 whereinsaid differential level signal output circuit responds to a choppedsignal so said signal levels represent the envelope of the choppedsignals, said first voltage generating means comprises a DC bridgecircuit having-a pair of DC voltage input terminals and a pair ofresistance-containing branches extending between said voltage inputterminals, there being associated with one of said branches saidtemperature measuring means which produces a variation in the voltagewith temperature between a point associated with said one branchconstituting an output terminal of the bridge circuit and a point incommon between said branches, said manually operable temperature setpoint means including means for varying the point of connection of asecond output terminal for the bridge to said other branch; second andthird capacitors respectively coupled between said bridge outputterminals and said common point; and said means for feeding a signal tothe input of said differential input signal responsive circuit includesmeans for coupling a voltage following that across said first mentionedcapacitor across the terminals of said second and third capacitorsremote from said common point, a fourth capacitor having one plateconnected to said common point, switch means for connecting the otherplate of said fourth capacitor alternately and cyclically to the plateof said second and third capacitors remote from said common point, andmeans for coupling the output of said fourth capacitor to the input ofsaid differential input responsive circuit.

12. The proportional temperature control apparatus of claim 2 whereinthere is provided offset eliminatingmeans comprising means forprogressively reducing said second input voltage component to zero overa prolonged period.

13. In proportional temperature control apparatus including an output atwhich heating or cooling producing signals are to be cyclicallyapplieds'o the proportion of each cycle a heating or cooling producingsignal is present is a maximum at one end of said temperature range andprogressively decreases to a minimum near zero at the other end of thetemperature range, manually operable temperature set point means forselecting the midpoint of the range of temperatures over which theproportional temperature control is to take place, and temperaturemeasuring means responsive to the temperature of the environment to beheated or cooled, the improvement comprising: a differential inputsignal responsive circuit which generates a heating or cooling producingoutput signal when a resultant input voltage Er reaches a first level V1and continues to generate a heating or cooling producing output signaluntil Er changes direction and reaches a second voltage level V2,whereupon said heating or cooling producing signal disappears until Eragain reaches V1; first signal input generating means responsive to saidtemperature measuring means and said manually operable temperature setpoint means for providing a first input voltage component ei whichprogressively varies between a third and fourth level Vpl and Vp2 as thetemperature of said environment varies between the limits of saidtemperature range over which proportional control is effected; andsecond voltage input generating means which provides a second inputvoltage component ei' coupled in additive relationship with said firstinput voltage component ei to the input of said differential inputvoltage responsive section, said second voltage input generating meansincluding a capacitor across which said second input voltage componentei is generated, when the temperature of said environment is at one endof said temperature range the variation in the capacitor voltage to makeEr vary between V1 and V2 being Vcl and Val respectively to initiate andterminate said heating or cooling input signal, and when the temperatureof said environment is at the other end of said temperature range thevariation in capacitor voltage to make Er vary between V1 and V2 beingVc2 and VcZ' respectively to initiate and terminate said heating orcooling output signal, means responsive to the presence of a heating orcooling signal in the output of the apparatus for applying to saidcapacitor through a charging resistance a voltage V3 which is at or nearthe value Vcl' so it will take a relatively long time for the charge onthe capacitor to reach the value Vcl' relative to the time it takes toreach the value Vc1 when the temperature of the environment is at ornear one end of said temperature range, and means responsive to theabsence of a heating or cooling signal in the output of the apparatusfor applying to said capacitor through a charging resistance a voltageV4 which is at or near the value V02 so it will take a relatively longtime for the charge on the capacitor to reach the value Vc2 relative tothe time it takes to reach the value V02 when the temperature oftheenvironment is at or near the other end of said temperature range.

14. The proportional temperature control apparatus of claim 13 whereinthe number of voltage units between said voltage levels V1 and V2 is afraction of the number.of voltage units between the voltages V3 and V4.

15. The proportional temperature control apparatus of claim 13 whereinsaid charging resistance for the capacitor remains substantiallyconstant.

16. The proportional temperature control apparatus of claim 13 whereinthere is provided means for selectively varying at least one of thevoltages V3 and V4 effectively simultaneously to vary the number ofdegrees in said temperature range over which proportional control iseffected and the period between the successive or cyclic appearance ofthe heating or cooling signals at the output of said apparatus, saidperiod increasing with the reduction in the number of degrees in saidtemperature range for any given relative position within the range.

17. The proportional temperature control apparatus of claim 13 whereinsaid means responsive to the presence and absence of the heating orcooling signals in the output of the apparatus including a bridgecircuit with the capacitor and 7 said charging resistance beingconnected across the output of the bridge circuit, and means for varyingthe relative values of the resistances in at least two arms of thebridge circuit in accordance with the presence and absence of theheating or cooling signals in the output of said apparatus to producesaid voltages V3 and V4.

18. The proportional I temperature control apparatus of claim 2 whereinsaid differential level signal output circuit responds to a choppedsignal so said signal levels represent the envelope of the choppedsignals, said first voltage generating means includes a bridge having apair of energizing voltage input terminals and a pair of resistancecontaining branches extending between said energizing voltage inputterminals, there being associated with at least one of said branchessaid temperature measuring means which produces a variation in thevoltage between a point associated with said one branch and constitutingan output terminal of the bridge circuit and a point in common with saidbranches, said manually operable set point means being associated withthe other of said branches to produce a variation in the voltage betweena point associated with the other branch and constituting another outputterminal of the bridge circuit and said common point, and a bridgeoutput circuit connected between said bridge output terminals whichincludes the voltage output of said second voltage generating meansapplied between a voltage addition point in said output circuit and saidcommon point so a voltage is provided at the voltage addition pointwhich is a func tion of the sum of the voltage at one of said bridgeoutput terminals and the output of said second voltage generating means,and said bridge output circuit including a chopper circuit comprising apair of switch means in series between said voltage addition point andthe other bridge output terminal, a capacitor coupled between thejuncture of said switch means and said common point, and means foralternately rendering said switch means conductive and nonconductive atthe desired chopping rate for alternately coupling said capacitorbetween said output terminals of the bridge circuit, the time constantof the circuit including said capacitor being such that the capacitorcharges to the applied voltage during the conduction period of theassociated switch means, and means for coupling the voltage variationson said capacitor to the input of said differential level signal outputcircuit.

19. The bridge circuit of claim 18 wherein said switch means are a pairof field effect transistors with the corresponding load terminalsthereof respectively connected between said capacitor and said voltageaddition point and the latter said other bridge input terminal, saidmeans for rendering said switch means alternately conductive includingmeans for feeding signals to the gate terminals of said field effecttransistors for alternately rendering the same conductive andnonconducnve.

1. A proportional temperature control apparatus for generating at anoutput heating or cooling producing signals to be cyclicallyintermittently applied over a range of temperatures so the proportion ofeach cycle a heating or cooling producing signal is present is a maximumat one end of said temperature range and progressively decreases to aminimum near zero at the other end of the temperature range, saidapparatus comprising: manually operable temperature set point means forselecting the midpoint of the range of temperatures over which theproportional temperature control is to take place, temperature measuringmeans responsive to the temperature of the environment to be heated orcooled, a control circuit responsive to said manually operabletemperature set point means and said temperature measuring means forgenerating said cyclically applied signals at said output within theselected temperature range, said control circuit including bandwidth andrate control means for simultaneously adjusting both the number ofdegrees included within said range of temperatures over which theproportional control is effected, and the rate at which said heating orcooling signals are cyclically applied to said output at correspondingpoints within the selected range of temperatures increasing with thenumber of degrees within the temperatures range over which proportionalcontrol is effected.
 2. In proportional temperature control apparatusincluding an output at which heating or cooling producing signals are tobe cyclically applied so the proportion of each cycle a heating orcooling producing signal is present is a maximum at one end of saidtemperature range and progressively decreases to a minimum near zero atthe other end of the temperature range, manually operable temperatureset point means for selecting the midpoint of the range of temperaturesover which the proportional temperature control is to take place, andtemperature measuring means responsive to the temperature of theenvironment to be heated or cooled, the improvement comprising: adifferential input signal responsive circuit which generates a heatingor cooling producing signal at the output of said apparatus when aninput signal fed thereto reaches a first level and continues to generatesaid heating or cooling producing signal at said output until said inputsignal reverses direction and reaches a second signal level, whereuponsaid heating and cooling producing signal disappears until the resultantinput signal again reaches said first level; first voltage generatingmeans responsive to said temperature measuring means and said manuallyoperable temperature set point means for providing a first input voltagecomponent which progressively varies between first and second voltagelevels as the temperature of said environment varies between the limitsof said temperature range over which proportional control is to beeffected; second voltage generating means which provides a second inputvoltage component which is mixed or added to said first input voltagecomponent; signal feeding means for feeding a signal to said input ofsaid differential input signal resPonsive circuit which is a function ofthe added or mixed voltage components; said second voltage generatingmeans including a capacitor across which said second input voltagecomponent is developed, and a charge circuit for the capacitor includingmeans for selectively providing first and second DC capacitor chargingvoltages having values spaced apart a number of voltage unitssubstantially greater than the variation of said second voltage inputcomponent, and means responsive to the presence of heating or coolingsignals in the output of said temperature control apparatus for causingthe capacitor to charge toward one of said DC voltages to bring thevoltage across the capacitor to a value which provides a resultantsignal at said second level at the input of said differential inputsignal responsive circuit which causes the heating or cooling signals todisappear from said output, and responsive to the absence of heating orcooling signals in the output of said temperature control apparatus forcausing the capacitor to charge toward the other of said DC voltages tobring the voltage across the capacitor to a value which provides aresultant signal at first level at the input of said differential inputsignal responsive circuit to reestablish said heating or cooling signalsat the output of said temperature control apparatus.
 3. The proportionaltemperature control apparatus of claim 2 wherein the time constant ofsaid capacitor charge circuit over the various temperatures involvedwithin said temperature range remains substantially constant.
 4. Theproportional temperature control apparatus of claim 2 wherein saidvoltage charge circuit of said second voltage generating means includingmeans for selectively varying the value of at least one of said firstand second DC capacitor charging voltages.
 5. The proportionaltemperature control apparatus of claim 2 wherein said means responsiveto the presence or absence of the heating or cooling signals in theoutput of the apparatus includes a bridge circuit having oppositebranches connected across a pair of voltage energizing input terminals,the capacitor and a charging resistance being connected acrossintermediate points of said branches constituting output terminals ofthe bridge circuit, and means for selectively varying the points ofconnection of said capacitor and charging resistance to at least one ofsaid branches in accordance with the presence and absence of the heatingor cooling signals in the output of said apparatus to provide said firstand said second capacitor charging voltages.
 6. The proportional controlapparatus of claim 5 wherein one of said branches has a potentiometerwith a wiper constituting one of said points of connection of saidcapacitor and charging resistance and the other point of connectionthereof in said one branch being a point other than the potentiometer,wherein the variation of the position of the wiper on the potentiometerwill progressively vary the magnitude of one of said capacitors chargingvoltages.
 7. The proportional control apparatus of claim 2 wherein saidmeans responsive to the presence and absence of the heating or coolingsignals in the output of the apparatus includes a voltage dividercircuit having at least three series connected resistor means extendingbetween a pair of DC energizing voltage input terminals, said capacitorhaving one plate coupled to a point in the voltage divider circuitremote from adjacent ones of said resistor means, and switch meansrespectively coupling the other plate of said capacitor through acharging resistor means to the remote ends of said adjacent resistormeans when the heating or cooling signals are respectively present andabsent in the output of the proportional control apparatus.
 8. Theproportional temperature control apparatus of claim 7 wherein said oneplate of said capacitor is coupled to the juncture of said adjacentresistor means through at least two series connected coupling resistormeAns of very high value relative to said capacitor charging resistormeans so the latter is insignificant relative thereto, and said meansfor feeding a signal to said differential input signal responsivecircuit including means responsive to the voltage across the one of saidseries connected coupling resistor means remote from said one plate ofsaid capacitor.
 9. The proportional temperature control apparatus ofclaim 8 provided with automatic offset eliminating means comprising asecond capacitor and a second charging resistor means connected inseries across the first-mentioned capacitor and its charging resistormeans, the time constant of the second capacitor and its chargingresistor means being extremely long relative to the time constant of thefirst-mentioned capacitor and its charging resistor means, and means forcoupling the voltage across said second capacitor to the terminal ofsaid one coupling resistor means remote from said first-mentionedcapacitor, whereby ultimately the voltage across said one couplingresistor means drops substantially to zero.
 10. The proportionaltemperature control apparatus of claim 8 wherein said first voltagegenerating means comprises a DC bridge circuit having a pair of DCvoltage input terminals and a pair of resistance-containing branchesextending between said voltage input terminals, there being associatedwith at least one of said branches variable responsive means whichproduces a variation in the voltage between a point associated with saidone branch constituting an output terminal of the bridge circuit and apoint in common between said branches, said manually operabletemperature set point means including means for varying the point ofconnection of a second output terminal for the bridge to said otherbranch; second and third capacitors respectively coupled between saidbridge output terminals and said common point; and said means forfeeding a signal to the input of said differential input signalresponsive circuit includes means for coupling the output across saidone coupling resistor means of said voltage divider circuit respectivelyacross the terminals of said second and third capacitors remote fromsaid common point, said voltage divider and bridge circuits sharing acommon DC voltage source, a fourth capacitor having one plate connectedto said common point, switch means for connecting the other plate ofsaid fourth capacitor alternately and cyclically to the plate of saidsecond and third capacitors remote from said common point, and means forcoupling the output of said fourth capacitor to the input of saiddifferential input responsive circuit.
 11. The proportional temperaturecontrol apparatus of claim 2 wherein said differential level signaloutput circuit responds to a chopped signal so said signal levelsrepresent the envelope of the chopped signals, said first voltagegenerating means comprises a DC bridge circuit having a pair of DCvoltage input terminals and a pair of resistance-containing branchesextending between said voltage input terminals, there being associatedwith one of said branches said temperature measuring means whichproduces a variation in the voltage with temperature between a pointassociated with said one branch constituting an output terminal of thebridge circuit and a point in common between said branches, saidmanually operable temperature set point means including means forvarying the point of connection of a second output terminal for thebridge to said other branch; second and third capacitors respectivelycoupled between said bridge output terminals and said common point; andsaid means for feeding a signal to the input of said differential inputsignal responsive circuit includes means for coupling a voltagefollowing that across said first mentioned capacitor across theterminals of said second and third capacitors remote from said commonpoint, a fourth capacitor having one plate connected to said commonpoint, switch means for connecting the oTher plate of said fourthcapacitor alternately and cyclically to the plate of said second andthird capacitors remote from said common point, and means for couplingthe output of said fourth capacitor to the input of said differentialinput responsive circuit.
 12. The proportional temperature controlapparatus of claim 2 wherein there is provided offset eliminating meanscomprising means for progressively reducing said second input voltagecomponent to zero over a prolonged period.
 13. In proportionaltemperature control apparatus including an output at which heating orcooling producing signals are to be cyclically applied so the proportionof each cycle a heating or cooling producing signal is present is amaximum at one end of said temperature range and progressively decreasesto a minimum near zero at the other end of the temperature range,manually operable temperature set point means for selecting the midpointof the range of temperatures over which the proportional temperaturecontrol is to take place, and temperature measuring means responsive tothe temperature of the environment to be heated or cooled, theimprovement comprising: a differential input signal responsive circuitwhich generates a heating or cooling producing output signal when aresultant input voltage Er reaches a first level V1 and continues togenerate a heating or cooling producing output signal until Er changesdirection and reaches a second voltage level V2, whereupon said heatingor cooling producing signal disappears until Er again reaches V1; firstsignal input generating means responsive to said temperature measuringmeans and said manually operable temperature set point means forproviding a first input voltage component ei which progressively variesbetween a third and fourth level Vp1 and Vp2 as the temperature of saidenvironment varies between the limits of said temperature range overwhich proportional control is effected; and second voltage inputgenerating means which provides a second input voltage component ei''coupled in additive relationship with said first input voltage componentei to the input of said differential input voltage responsive section,said second voltage input generating means including a capacitor acrosswhich said second input voltage component ei is generated, when thetemperature of said environment is at one end of said temperature rangethe variation in the capacitor voltage to make Er vary between V1 and V2being Vc1 and Vc1'' respectively to initiate and terminate said heatingor cooling input signal, and when the temperature of said environment isat the other end of said temperature range the variation in capacitorvoltage to make Er vary between V1 and V2 being Vc2 and Vc2''respectively to initiate and terminate said heating or cooling outputsignal, means responsive to the presence of a heating or cooling signalin the output of the apparatus for applying to said capacitor through acharging resistance a voltage V3 which is at or near the value Vc1'' soit will take a relatively long time for the charge on the capacitor toreach the value Vc1'' relative to the time it takes to reach the valueVc1 when the temperature of the environment is at or near one end ofsaid temperature range, and means responsive to the absence of a heatingor cooling signal in the output of the apparatus for applying to saidcapacitor through a charging resistance a voltage V4 which is at or nearthe value Vc2 so it will take a relatively long time for the charge onthe capacitor to reach the value Vc2 relative to the time it takes toreach the value Vc2'' when the temperature of the environment is at ornear the other end of said temperature range.
 14. The proportionaltemperature control apparatus of claim 13 wherein the number of voltageunits between said voltage levels V1 and V2 is a fraction of the numberof voltage units between the voltages V3 and V4.
 15. The proportionaltemperature control apparatus of claim 13 wherein said chargingresistance for the capacitor remains substantially constant.
 16. Theproportional temperature control apparatus of claim 13 wherein there isprovided means for selectively varying at least one of the voltages V3and V4 effectively simultaneously to vary the number of degrees in saidtemperature range over which proportional control is effected and theperiod between the successive or cyclic appearance of the heating orcooling signals at the output of said apparatus, said period increasingwith the reduction in the number of degrees in said temperature rangefor any given relative position within the range.
 17. The proportionaltemperature control apparatus of claim 13 wherein said means responsiveto the presence and absence of the heating or cooling signals in theoutput of the apparatus including a bridge circuit with the capacitorand said charging resistance being connected across the output of thebridge circuit, and means for varying the relative values of theresistances in at least two arms of the bridge circuit in accordancewith the presence and absence of the heating or cooling signals in theoutput of said apparatus to produce said voltages V3 and V4.
 18. Theproportional temperature control apparatus of claim 2 wherein saiddifferential level signal output circuit responds to a chopped signal sosaid signal levels represent the envelope of the chopped signals, saidfirst voltage generating means includes a bridge having a pair ofenergizing voltage input terminals and a pair of resistance containingbranches extending between said energizing voltage input terminals,there being associated with at least one of said branches saidtemperature measuring means which produces a variation in the voltagebetween a point associated with said one branch and constituting anoutput terminal of the bridge circuit and a point in common with saidbranches, said manually operable set point means being associated withthe other of said branches to produce a variation in the voltage betweena point associated with the other branch and constituting another outputterminal of the bridge circuit and said common point, and a bridgeoutput circuit connected between said bridge output terminals whichincludes the voltage output of said second voltage generating meansapplied between a voltage addition point in said output circuit and saidcommon point so a voltage is provided at the voltage addition pointwhich is a function of the sum of the voltage at one of said bridgeoutput terminals and the output of said second voltage generating means,and said bridge output circuit including a chopper circuit comprising apair of switch means in series between said voltage addition point andthe other bridge output terminal, a capacitor coupled between thejuncture of said switch means and said common point, and means foralternately rendering said switch means conductive and nonconductive atthe desired chopping rate for alternately coupling said capacitorbetween said output terminals of the bridge circuit, the time constantof the circuit including said capacitor being such that the capacitorcharges to the applied voltage during the conduction period of theassociated switch means, and means for coupling the voltage variationson said capacitor to the input of said differential level signal outputcircuit.
 19. The bridge circuit of claim 18 wherein said switch meansare a pair of field effect transistors with the corresponding loadterminals thereof respectively connected between said capacitor and saidvoltage addition point and the latter said other bridge input terminal,said means for rendering said switch means alternately conductiveincluding means for feeding signals to the gate terminals of said fieldeffect transistors for alternately rendering the same conductive andnonconductive.